1.Field of the Invention
This invention relates to a fuel injection system for an internal combustion engine for controlling a fuel amount to be supplied to a combustion chamber so that optimum combustion conditions can be obtained, especially when starting the engine.
2. Prior Art
Various types of fuel injection systems have been proposed for realizing stable starting of an internal combustion engine before the engine has been warmed up. One example is an electronic fuel injection system disclosed in Japanese laid open patent publication No. 61-215428. In this prior art, when the internal combustion engine starts, the fuel injection is controlled so that a preset amount of fuel, being larger than the amount to be supplied after complete combustion occurs, is, constantly supplied until the rotational speed of the engine reaches a predetermined value. After the rotational speed of the engine reaches the predetermined value, the fuel injection amount to be injected is gradually decreased to approach an amount which is determined based on an air-fuel ratio control for post-start state as shown by a single-dash broken line I in FIGS. 8 and 9.
If the increased amount of fuel for starting the engine is constantly supplied for a long time, the rotational speed of the engine may increase more than expected, resulting in deteriorating fuel economy, as indicated by the double-dash broken line IIA in FIGS. 8 and 9. If the environmental temperature of the engine is extremely low, the rotational speed of engine may not reach the target rotational speed corresponding to the complete combustion state. As a result, the air-fuel ratio becomes "over rich", which is apt to deteriorate the starting efficiency of the engine. This condition is indicated by the broken line IIB in FIGS. 8 and 9. On the other hand, if the fuel injection amount is suddenly changed from the amount based on an engine start control to that based on an air-fuel ratio control for post-start state before the rotational speed of the engine reaches the target rotational speed corresponding to stable combustion, the fuel supply amount may become insufficient. As a result, the rotational speed of the engine is suddenly decreased as shown by a broken line III in FIGS. 8 and 9, and so-called engine stall may happen.
The above-mentioned prior art fuel injection system (No. 61-215428) has solved those problems. However, it still includes other problems as follows. As described above, the fixed fuel injection amount during starting the engine is gradually decreased to an amount determined by the air-fuel ratio control for post-start state. Since the fuel injection amount determined by the air-fuel ratio for post-start state varies, the excess of fuel injection amount over the amount determined by the air-fuel ratio for post-start state changes during a period from time point T1, at which the complete combustion condition is about to start, to time point T2, at which the air-fuel ratio control is started. For example, if the fuel injection amount during idling under the complete combustion condition is small, the excess fuel amount to be supplied during the period of decreasing the fuel injection amount (shown by the shaded portion in FIG. 8) may exceed a required amount. As a result, the air-fuel ratio becomes "over rich" before the fuel injection amount based on the engine start control is changed over to the amount based on the air-fuel ratio control, for post-start state. Under such a condition, not only is the fuel wasted but also hydrocarbon (HC) contained in the exhaust gas increases.